The present invention relates generally to durability testing of test specimens such as but not limited to, artificial orthopedic implants (e.g. hip, knee, spine, etc.). More specifically, the present invention pertains to a system and method for combining measured signals with virtual signals generated by a model to extend the range of mechanical methods of load testing.
Laboratory simulation is a technique that is often used to validate the durability of orthopedic implant designs and to verify manufacturing quality assurance. In the case of the artificial knee joint, it is desirable to place the Anterior-Posterior and Tibial-Rotation degrees of freedom in load or torque control, while at the same time controlling the load in the vertical degree of freedom. Pure displacement control in these directions is deficient because it does not account for the changes in specimens over time or variation between specimen designs (e.g. levels of constraint). In addition, pure load control is made difficult by the variation in constraint levels between specimens and within a given specimen over its operating range (e.g. transitions from static to kinetic friction states, collision with hard mechanical limits built into the specimen, etc.). Previous research exists for the force inputs into the body, relative displacements expected in a healthy joint and the behavior of the surrounding soft tissue. It is also known that over time, as specimens wear, constraint levels and coefficients of friction change. It is not feasible to do long-term durability tests that incorporate all aspects of the in-vivo environment, such as, the living soft tissue. Therefore, commonly utilized test systems face an increasingly difficult task of applying forces to specimens that adequately emulate realistic conditions in addition to compensation techniques in the event of partial failure of the specimen. Design of simulators to apply varying loads is complicated by cross talk between channels and the continuously varying nature of each programmed load. Further difficulties arise from the large variation in implant design and the associated widely varying degrees of joint constraint.
Once an artificial joint is implanted, it is constrained by a combination of mechanical interlock, frictional forces and the soft tissue surrounding the joint. The mechanical interlock and frictional forces may be directly replicated in the specimen, whereas the soft tissue is more difficult to simulate.
Soft tissue reaction forces have been implemented in orthopedic simulators in the past by use of mechanical springs which have significant disadvantages including limited durability, difficulty in changing values, limited mathematical nature of the reaction forces, difficulty in attaining appropriate configuration and overall complexity of the machine.
Therefore, there is a significant need to improve systems that are used to test specimens for durability and other factors. A system that addresses one or more of the shortcomings discussed above would be particularly useful.